Burying rusty nails adds iron that greens up plants and turns hydrangeas blue.
Rusty nails won't green up your plants or turn hydrangeas blue. Yellow leaves usually mean high pH has locked the iron up. It isn't missing, and iron from a nail gets locked up the same way. Hydrangea blue comes from aluminum in acidic soil, not iron. Soil-test first; for yellowing, sulfur (where there's no free lime) or an iron leaf spray as a stopgap; to blue a hydrangea, add sulfur to the soil or use aluminum sulfate.
What is actually known
Two separate claims ride on this, and both miss. Yellowing leaves (chlorosis) usually don't mean a shortage of iron in the soil, which typically holds plenty. The cause is iron that a high pH has turned into solids the roots cannot take up. Iron released from rusty nails or shavings forms those same unavailable solids at once, so it does nothing for the yellowing. Hydrangea color is not an iron effect either. Blue comes from aluminum, which acidic soil makes available to the roots and neutral or alkaline soil locks away. Start with a soil test. Lowering pH with sulfur is worth trying only where the soil has no free lime, and iron sprays on the leaves act fast but are spotty and temporary. To blue a hydrangea, work sulfur into the soil or water it with aluminum sulfate.
The evidence
Evidence: checked against 3 sources in the research-based literature.
The 3 sources
Even though there typically is plenty of iron in these soils, the high soil pH causes chemical reactions that make the iron solid and unavailable to plant roots
Soil applications of iron alone are ineffective
The color variation is due to the presence or absence of aluminum compounds in the flowers
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Cite this page
Milpa Gardens. "Burying rusty nails adds iron that greens up plants and turns hydrangeas blue." Evidence grade: Refuted. https://milpa.garden/beliefs/rusty-nails-iron-plants/
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